The present invention relates to a process for preparing pre-expanded particles of a thermoplastic resin, and more particularly to in a process for preparing pre-expanded particles of a thermoplastic resin which comprises releasing into a low pressure zone an aqueous dispersion of thermoplastic resin particles containing a volatile blowing agent, the dispersion being maintained at an elevated temperature under pressure in a pressure vessel, the improvement which comprises releasing the dispersion through an orifice having a ratio of the length to the average diameter of 4 to 100.
The pre-expanded particles of thermoplastic resin obtained according to the process of the present invention are uniform and large in diameter of cells, and accordingly have excellent expansion moldability in a mold.
As to process for pre-expanding particles of thermoplastic resins, there have hitherto been adopted processes disclosed in, for instance, Japanese Examined patent publication No. 1344/1981, DE-OS No. 2,107,683, and the like, and there have been obtained pre-expanded particles which can be used in expansion molding in a mold by means of the above processes.
However, the cells of the pre-expanded thermoplastic resin particles prepared by the above processes have generally a diameter of not more than 200 .mu.m. When the expansion molding is carried out in a mold by using the pre-expanded particles having a cell diameter of not more than 200 .mu.m, the obtained molded articles have defects that the shrinkage is large and their shapes are strained, which are generally known as "shrinkage or depression" or "camber" in the art. Accordingly, the value of products and the productivity of the molded articles are lowered.
It is an object of the present invention is to solve the above mentioned disadvantages.
The above and other objects of the present invention will become apparant from the description hereinafter.